Preheat the oven to 400°F. Whip the egg whites to soft peaks adding salt and pepper to taste.

In a heavy 9-inch frying pan that can go into the oven, melt the butter over high heat. Add the mushrooms, season lightly and cook until the mushrooms become soft but not brown. Arrange the tomato over the mushrooms. Quickly fold the cheese into the whites and spread out evenly over the mushroom mixture. Place in the oven and bake for 8 minutes until golden on top.

Remove from the oven and run a spatula around the sides to loosen. Flip over onto a plate and serve immediately and only 6 carbs per serving.